about use converters matplotlib.dates.datestr2num in numpy.loadtxt

mean-temperature-graph_all.py

#!/usr/bin/env python

"""Draw line graph of 118 years August temperature line graph.

Past data of temperature gets from
https://www.data.jma.go.jp/gmd/risk/obsdl/index.php

Sample original data
2018,8,8,30.3,8,2,35.1,8,2,24.8,8,2
2018,8,9,31.2,8,2,37.3,8,2,26.7,8,2
2018,8,10,30.5,8,2,35.4,8,2,27.3,8,2
2018,8,11,29.8,8,2,34.1,8,2,26.1,8,2
2018,8,12,30.4,8,2,34.7,8,2,27.2,8,2

Replace two times
s/(\d{4}),8,(\d{1})/'$1-08-0$3'/
s/(\d{4}),8,(\d{2})/'$1-08-$3'/

Imput data
'2018-08-08',30.3,8,2,35.1,8,2,24.8,8,2
'2018-08-09',31.2,8,2,37.3,8,2,26.7,8,2
'2018-08-10',30.5,8,2,35.4,8,2,27.3,8,2
'2018-08-11',29.8,8,2,34.1,8,2,26.1,8,2
'2018-08-12',30.4,8,2,34.7,8,2,27.2,8,2
"""
from matplotlib import pyplot as plt
from matplotlib import dates as mdates
import numpy as np


dt = np.dtype({
    'names': ['date', 'mean', 'max', 'min'],
    'formats': ['f8', 'f4', 'f4', 'f4']
})

# Matplotlib represents dates using floating point numbers specifying the
# number of days since 0001-01-01 UTC, plus 1.
# For example, 0001-01-01, 06:00 is 1.25, not 0.25.
# Values < 1, i.e. dates before 0001-01-01 UTC are not supported.
# Reading data converting date string number.
# My sample data can't work `converters={0: mdates.datestr2num}`,
# becase '2018-08-01' is Bytes literals b'2018-08-01'.
# c.f. https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals 
datas = np.loadtxt('data/august-temperature.csv', dtype=dt, skiprows=1,
                   delimiter=',',
                   converters={0: lambda s: mdates.datestr2num(s.decode())},
                   usecols=(0, 1, 4, 7))
# print(datas)

# round to nearest years.
datemin = np.datetime64(mdates.num2date(datas['date'][0]), 'Y')
datemax = np.datetime64(mdates.num2date(
    datas['date'][-1]), 'Y') + np.timedelta64(1, 'Y')
# print(datemax, datemin)

fig, ax = plt.subplots()
ax.set_xlim(datemin, datemax)

# format the cords message box
ax.format_xdata = mdates.DateFormatter('%Y')
ax.format_ydata = lambda x: '$%2.1f' % x  # format the temperature.
ax.grid(True)

ax.plot('date', 'max', data=datas)

# rotates and right aligns the x labels, and moves the bottom of the
# axes up to make room for them
fig.autofmt_xdate()

plt.title('every August Max temperature at Tokyo from 1901 to 2018.')
plt.show()